The invention relates to an extruder for the processing and production of rubber and thermoplastic materials. The invention comprises an improvement of the extruder disclosed in application Ser. No. 07/693,225, filed Apr. 29, 1991 and now U.S. Pat. No. 5,127,741, which is incorporated herein by reference.
In the past, widespread use has been made of so-called pin-lined barrel extruders, such as are known from German Offenlegungsschrift 2,235,784 or German Offenlegungsschrift 3,003,615, as discharging and homogenizing extruders. In extruders of this type, metal pins protrude radially through the extruder barrel into the processing space of the extruder, the extruder screw having interrupted screw flights in this zone to accommodate the pins.
These extruders are distinguished by a very high output rate and good homogenizing effect on the material to be processed and, in addition, permit an increased material throughput per unit time at the same screw speed in comparison with conventional cold-feed extruders with a screw having a shear section These advantages have resulted in pin-lined barrel extruders becoming the most frequently used extruders in the rubber industry in the last 15 years.
Independently of this, an extruder mixing section which has become known as the transfer mixing section has been developed and is disclosed in DE-A 1,142,839. This mixing section essentially comprises the feature that both the extruder screw and the inner wall of the extruder barrel are provided with grooves and flights over a certain length, the channel depth of the extruder screw decreasing to zero and subsequently increasing again in the longitudinal direction of the extruder barrel to the same degree as the channel depth of the barrel grooves respectively increases and decreases again. Due to this design of extruder screw and barrel, a complete extrudate exchange between the screw grooves and the barrel grooves is possible, which causes a good mixing effect.
The transfer extruder was able to claim a certain share of the market over the pin-lined barrel extruder, in particular, whenever the overall length of the 10 extruder was to be kept small.
In addition, it is known from U.S. Pat No. 3,613,160 to provide extruders with flow restricting components, by which the extrudate transport in the extruder can be variably controlled from the outside. According to this publication, an approximately cylindrical component is arranged for this purpose on the screw shank of the extruder screw, which component rotates along with the screw and completely blocks the processing space downstream. In the region of the cylindrical component, two restricting pins are, in each case, led through the extruder barrel from outside radially into an axially directed overflow channel, which is formed in the inner wall of the extruder barrel. With retracted restricting pins, part of the extrudate upstream of the cylindrical component can pass through these channels to the downstream extruder section. By moving the restricting pins into these overflow channels to different depths, this extrudate stream can be restricted.
In addition, German Patent 2,731,438 discloses a transfer extruder in which the number of flights in the barrel and on the screw in the transfer zone reduces as the cross-sectional area of the screw channels becomes greater, and increases as the cross-sectional area of the screw channels becomes smaller. In this case, screw channels of greater cross-sectional area have a greater width than screw channels of smaller cross-sectional area.
The tests carried out in the past on such mixing sections have indeed shown that comparatively good mixing qualities can be achieved with them, and that the mixing section is self-cleaning. However, what was disadvantageous was that such designs required a relatively high manufacturing outlay, for which reason the production costs were high.
Finally, it is known from German Offenlegungsschrift 3,133,708 and European Patent 0,062,203 to provide the inner side of the extruder barrel of single screw extruders with axial or helical barrel grooves in the feed zone. Such extruders are generally used for the extrusion of polymer materials of high melt viscosity, low thermal stability or low surface friction with respect to the metal surfaces of the extruder processing space. Such an extruder allows a high output rate at low melt temperatures and high melt pressures even in the case of those such extrudates which are difficult to process. In addition, due to the reduced shearing in the feed zone of the extruder, the extrudate can be plasticized gently and extruded surge-free.